Pathogenic organisms, including viruses, may be divided into different classes on the basis of their fate after being phagocytized. For instance, organisms that are promptly destroyed when phagocytized (i.e., S. pneumoniae, S. pyogenes) behave as extracellular parasites, damaging tissues only so long as they remain outside phagocytic cells.
Pathogens that function as extracellular parasites owe their virulence to antiphagocytic surface components. Most pathogenic bacteria for example, maintain capsules comprising high-molecular-weight polysaccharides. The relation between capsules, phagocytosis and virulence is clearly exemplified by S. pneumoniae. A fully encapsulated S (smooth) strain is found to resist phagocytosis (in the absence of antibodies) and is highly virulent for mice, whereas its nonencapsulated R (rough) mutant is readily phagocytized and is essentially avirulent. However, enzymatic removal of the capsular polysaccharide, or combination with antibody, renders the S organisms both nonpathogenic and susceptible to phagocytosis.
In contrast to extracellular parasites, there are two classes of intracellular parasites, both of which can multiply within phagocytic cells. One class of these intracellular pathogens comprise the obligate intracellular parasites (i.e., rickettsiae, chlamydiae). For these organisms, which include viruses, subtle differences in the cell surface receptors essential for their uptake may be important in resistance.
Many pathogens, whether intracellular or extracellular, are organotropic. That is, they are highly selective in regard to the tissue or cell-type that they infect or invade. One determinant of tissue tropism is the presence of surface macromolecules on the pathogen that promote adherence to specific receptors on one host cell-type but not on others. For instance, the role of specific bacterial adherence is now increasingly recognized in the selective colonization of host tissue or cell-types. For many obligate intracellular parasites, the molecules responsible for organotropic adherence are also crucial to the internalization of the pathogen.
Blocking this cell-specific interaction aids in the destruction of pathogenicity. For instance, antibodies to pathogen surface constituents promote immunity not only by opsonization but also by covering antigens involved in adherence. Moreover, in body secretions, secreted glycoproteins closely related to cell surface components may play a role in host defense by competing with fixed receptors and thus preventing adsorption of adherent pathogens.
Viruses, which are noncellular in nature, are vastly less complex than prokaryotic or eukaryotic cellular systems. Although as a group they are extremely heterogeneous, all viruses share certain basic properties. All viruses are obligate intracellular parasites; i.e., they cannot reproduce unless present within some host cell. Outside of the host cell, the virus exists as a particle, or virion, in which its genetic material is enclosed within a capsid shell comprising protein subunits, and in some instances, a membranous envelope as well.
Viruses have been demonstrated to exhibit cell-specific tropism in that a given virus will infect only certain cell-types. The host cell range of the virus is determined by the specificity of attachment to the cells, which depends on properties of both the virion's capsid and specific receptors on the cell surface. The influenza viruses, for example, have as part of their capsid structure the hemagglutinin protein which facilitates the binding of the virus to receptors present on host cells. These limitations disappear when transfection occurs, i.e., when infection is carried out by the naked viral nucleic acid, whose entry does not depend on virus-specific receptors.
CD4, a surface glycoprotein found primarily on a subset of T lymphocytes, is a receptor for both the class II major histocompatability complex (MHC) antigens and the human immunodeficiency viruses (HIV). The tropism of the HIV virus for CD4+ cells is governed through the direct binding and high affinity interactions between virion-associated gp120 protein and CD4 (McDougal et al. (1986) Science 231:382 and Lasky et al. (1987) Cell 50:975). In addition, cells expressing the envelope protein fuse with CD4-bearing cells in culture (Lipson et al. (1986) Nature 323:725 and Sodroski et al. (1986) Nature 322:470), resulting in the formation of multinucleate syncytia.
The most amino-terminal immunoglobulin-like domain of CD4 is sufficient to bind gp120, although the second domain has also been shown to contribute to binding (Traunecker et al. (1988) Nature 331:84; Berger et al. (1988) PNAS 85:2357; Richardson et al. (1988) PNAS 85:6102 and Clayton et al. (1988) Nature 335:363). Additionally, studies of carbohydrate-mediated interactions of the envelope glycoprotein, gp120, have presented evidence that binding to cell surfaces may also involve the carbohydrate moieties of gp120 (Larkin et al., 1989 AIDS 3:793). Antiviral therapies directed towards abrogating proliferation of the HIV virus have been directed to specifically inhibiting interactions between CD4 and gp120. Strategies have included antibodies directed against various epitopes on gp120, facilitated for instance by immunization with immunogenic peptides resembling portions of gp120 or administration of anti-gp120 monoclonal antibodies. Both native and recombinant gp120 elicit antibodies that are capable of neutralizing HIV in cell culture (Robey et al. (1986) PNAS 83:9709; Laskey et al. (1986) Science 233:209 and Putney et al. (1986) Science 234:1392). These antibodies however are generally neutralizing only to the variant from which the immunizing gp120 was derived.
Studies in humans and mice have revealed a small region of gp120, termed the V3 loop or principal neutralizing determinant, comprising about 35 residues between two invariant, disulfide-crosslinked cysteines (Cys-303 to Cys-338: HIV-1 nomenclature of Takahashi et al. (1992) Science 255:333), that evokes the major neutralizing antibodies to the virus (Palker et al. (1988) PNAS 85:1932; Rusche et al. (1988) PNAS 85:3198 and Goudsmit et al. (1988) PNAS 85:4478). While this same region is one of the most variable in sequence among different clonal isolates (Takahashi et al. (1992) Science 255:333), analysis of the amino acid sequences of this domain revealed conservation to better than 80-percent of the amino acids in 9 out of 14 positions in the central portion of the V3 loop, suggesting that there are constraints on the V3 loop variability (LaRosa et al. (1990) Science 249:932). These results suggest that HIV vaccine immunogens chosen because of their similarity to the consensus V3 loop sequence and structure are likely to induce antibodies that neutralize a majority of HIV isolates.
Soluble forms of CD4 have been shown to be capable of inhibiting the interaction between HIV gp120 and CD4+ cells, presumably by binding and masking gp120 on the surface of the virus or infected cell (Traunecker et al. (1989) Nature 339:68; Fisher et al. (1988) Nature 331:76; Chao et al. (1989) JBC 264:5812 and Capon et al. (1989) Nature 337:525). Compounds such as Dextran sulphate and aurintricarboxylic acid, which act to bind CD4 and thereby inhibit gp120 binding, have been explored for use as HIV prophylaxis (Schols et al. (1989) PNAS 86:3322 and Lederman et al. (1989) J Immunol. 143:1149). Recently, N-carboxymethoxy-carbonyl-prolyl-phenylalanyl benzyl esters have been shown to irreversibly denature gp120 in such a manner as to abrogate binding to CD4 and inhibit HIV-1 infection (Finberg et al. (1990) Science 249:287).
The use of toxins targeted to HIV infected cells has also been explored. For instance, the gp120-binding domain of CD4 has been linked to the Pseudomonas exotoxin A molecule such that cells expressing the HIV gp120 molecule are selectively destroyed (Chaudhary et al. (1988) Nature 335:369). Immunotoxins comprising human monoclonal antibodies specific for epitopes of the envelope proteins of HIV, gp120 or gp41, conjugated to toxins such as ricin A chain or diphtheria toxin, have been employed to specifically kill HIV infected cells (Till et al. (1989) PNAS 86:1987). Alternatively, pokeweed mitogen, targeted to CD4+ cells has been shown to efficiently block HIV protein synthesis and also strongly inhibit HIV production in activated CD4+ T cells from infected patients (Zarling et al. (1990) Nature 347:92).
It has recently been reported that located close to the crown of the V3-type specific neutralization loop of the HIV-1 virus, are several potential sites that are susceptible to proteolytic cleavage by enzymes of trypsin-like or chymotrypsin-like specificity, or by aspartic proteases. (See for example, Clements et al., 1991, AIDS Research and Human Retroviruses 7:3; Hattori et al., 1989, FEBS Letters 248:48; Kioto et al., 1989, INT Immunol. 1:613; Stephens et al., 1990, Nature 343:219., incorporated by reference herein.)